: A JOURNEY TO THE BEGINNING OF THE

C.T. Russell(1), A. Coradini(2), W.C. Feldman(3), R. Jaumann(4), A.S. Konopliv(5), T.B. McCord(6), L.A. McFadden(7), H.Y. McSween(8), S. Mottola(4), G. Neukum(9), C.M. Pieters(10) C.A. Raymond(5), D.E. Smith(11), M.V. Sykes(12) B.G. Williams(5), and M.T. Zuber(13)

1IGPP&ESS, UCLA, Los Angeles, CA 90095-1567; [email protected] 2IFSI, Via del fosso del Cavaliere, 00133 ROMA Italy 3LANL, MS D466, NIS-1, Los Alamos, NM 87545 4DLR Rutherfordstr 2, D-12489 Berlin, Germany 5JPL, 4800 Oak Grove Dr., Pasadena, CA 91109 6University of Hawaii, 2525 Correa Rd., Honolulu, HI 96822 7University of Maryland, College Park, MD 20742 8University of Tennessee, Knoxville, TN 37996-1410 9Freie Universitat Berlin, Malteser Str.74-100, 12249 Berlin, Germany 10Brown University, Providence, RI 02912 11GSFC, MC 920, Greenbelt, MD 20771 12U of Arizona, Tucson, AZ 85721 13MIT, Cambridge, MA 02139

ABSTRACT/RESUME Once formed, by gravitational collapse, its orbital motion relative to the Dawn, NASA's ninth Discovery mission, is provided a periodic gravitational pulse that scheduled to launch on May 27, 2006 on a accelerated them so they would no longer journey that will take it into about the two accrete. Thus the formation of Jupiter stopped most massive asteroids and 1 . much of the geochemical evolution of this Dawn's goal is to understand the conditions and region. By exploring the two most massive processes present at the solar system's earliest planetesimals, Dawn studies some of the earliest , and the role of water content and size in processes in the solar system, those occurring planetary evolution. To this end Dawn carries a during its first 5-10 million years. framing camera, a mapping spectrometer, a laser altimeter, a gamma-ray/neutron spectrometer, a 2. THE MISSION magnetometer and a gravity investigation. Dawn uses solar arrays to power its xenon ion engine Figure 1 introduces the Dawn mission. The that provides thrust at an efficiency that is ten backdrop is a painting by W.K. Hartmann of a times greater than chemical rockets provide. nebula as the central begins to shine. Visible Dawn is a partnership between UCLA, JPL, and are the dust and rocks that are coming together to the American, German and Italian space form the terrestrial or rocky . Also shown agencies. are the , 4 Vesta and 1 Ceres as we see them today with . 1. INTRODUCTION Vesta is about 520 km across and Ceres almost 1000 km. Vesta has a very bright surface and is Over 4.6 billion years ago a cold gas cloud, 2.34 AU from the . Ceres at 2.77 AU is much containing the material now found in the solar darker, consistent with the general darkening of system, began to collapse. In that cloud two material with distance from the sun in this region competing processes took place: a chemical of space. process, in which solid material formed out of the gases, making rocks and small planetesimals; The spacecraft has rather large solar panels and a physical process, in which formed by roughly 21 m tip-to-tip, a 5 m magnetometer a gravitational instability. Hot stars, once boom and three ion thrusters, one of which is formed, dispersed the gas cloud and stopped the shown in operation. The communication dish is formation of rocky planetary materials. In the aimed at the by pointing the entire region that we now call the belt, the spacecraft. When the instrument panel on the top process was stopped in yet another of the spacecraft has to face the body, the competition between physics and chemistry. spacecraft is oriented in this direction (nadir Fig. 1. Artist's conception of Dawn spacecraft thrusting its ion engines on the way to Vesta and Ceres. pointing). To undertake such a mission is were found by the Von Zach search team, difficult and expensive. In order both to employ including Vesta discovered by H. Olbers on relevant and recent heritage and to afford to be March 29, 1807. able to visit each of Vesta and Ceres, a partnership was formed between the American, We now know much about Ceres and Vesta. German and Italian space agencies. The German They are two of three remaining large asteroids, space agency, DLR, is represented by the the other being Pallas, which is much more Institute of Space Sensor Technology and difficult to reach. Tables 1 and 2 give some of Planetary Exploration in Berlin and the Italian the physical parameters of these bodies. Pallas is space agency, ASI, by CNR/IFSI in Rome. The dark, resides in the same region of the asteroid project is managed by the Jet Propulsion belt as Ceres and has relatively low density Laboratory for NASA and the spacecraft is built approaching that of Ceres. Although by Orbital Sciences Corporation. The overall measurements at Pallas are highly desirable, direction of the project is provided by the UCLA exploring Vesta and Ceres may be sufficient to principal investigator, C. T. Russell, with the bracket the properties of large "minor" planets. advice of the science team, the co-authors of this report. The reflectance spectrum of Vesta, contrasted to that of the other asteroids in Figure 2, is similar 3. TARGETS to that of the Howardite-Eucrite-Diogenite and to terrestrial basalts. The Late in the 18th century it was realized that the existence of basalts on the surface of Vesta Titius-Bode's law predicted a near 2.8 AU indicates that it melted at least partially. This where none had been seen. Thus a search was belief was later reinforced by geochemical initiated by Baron Von Zach, the court analyses of the HED meteorites that indicate that astronomer at Gotha. His consortium was a core formed. It appears that Vesta melted and scooped, however, on the first of the resolidified creating a layered structure with nineteenth century when of lighter crust on the outside, then pyroxene and Palermo discovered Ceres. Later other asteroids , and iron in successively deeper layers.

Table 1. Physical properties of the three largest asteroids

Body Principal Density Period G IRAS [1019kg] Radii [km] [kg/m3 ] [hr] [m/s2] Albedo 1 Ceres 94.8 ± 0.3 480x480x454 2200 ± 50 9.08 0.23 0.11 2 Pallas 21.4 ± 0.8 287x263x251 2700 ± 150 7.81 0.30 0.16 4 Vesta 26.7 ± 0.3 280x272x227 3700 ± 100 5.34 0.32 0.42 1.2 Table 2. Orbital parameters of the largest asteroids Ceres, joined spectrum

Re-scaled NH4-montmorillonite Body SMA Eccent- Inclin- Period ricity ation 1.0 1 Ceres 2.77 AU 0.077 10.6 4.61 yrs 2 Pallas 2.77 AU 0.232 34.8 4.61 yrs 4 Vesta 2.36 AU 0.090 7.1 3.63 yrs 0.8

Ceres is very different than Vesta. It has not been Scaled Reflectance linked to any specific meteorites because in part it has a very bland spectrum. It is much darker 0.6 than Vesta and much less dense. It is believed to contain much water: on cosmological grounds, 2.4 2.6 2.8 3.0 3.2 3.4 3.6 based on its average density, and from evidence Wavelength (mm) for the alteration of surface materials by water in Fig. 3. IR reflectance of Ceres showing a spectral the spectrum shown in Figure 3. If Ceres could feature possibly associated with the effects retain a lot of water then it might have remained aqueous alteration [2] cool and unmelted with very little thermal alteration since formation. propulsion is performed using the ion thrusters and 400 kg of xenon fuel. The ion thrusters 4. MISSION DESIGN accelerate the xenon to a speed ten times that of chemical engines, enabling the mission to be Dawn's launch is scheduled to take place on May carried out with a small launch vehicle, keeping 27, 2006 on a Delta 2925H launch vehicle. costs within the Discovery guidelines. When the Figure 4 shows the interplanetary trajectory, spacecraft arrives at each object it is moving very including thrusting and non-thrusting periods. slowly because it fires its ion engines to decelerate Dawn arrives at Vesta four years after launch in for many months prior to performing a search of July 2010 and spends 11 months in orbit there. In the sky around the body for and dust. June 2011 it spirals outward from Vesta and When this survey is complete, the orbit is lowered heads to Ceres arriving three years later in to an approximately 700 km altitude orbit and the August 2014. Again it spends 11 months surface completely mapped with the framing mapping. All interplanetary and most planetary camera.

2006 Vesta-Ceres Rendezvous A

16

Vesta 3

8 Flora Earth

2 Vesta 4 Vesta Arrival 7/30/10 Launch 5/27/06 Normalized Spectral Reflectance 1 Ceres 1 7/3/11 8/20/14 Vesta Ceres Departure Arrival Ceres 0.5 1.0 1.5 2.0 2.5 µ Wavelength ( m) Non thrusting Thrusting Earth, Vesta, Ceres

Fig. 2 Visible and near IR reflectance spectrum of Vesta, Ceres and several other major Fig. 4. The planned Dawn interplanetary asteroids[1] trajectory Table 3. Orbital parameters of the Dawn spacecraft at Vesta and Ceres

Body Period Dist. Altitude FOV1 Pixel1 Pointing [hr] [km] [km] [km] [m] Optimized for Vesta 12.1 950 661-721 69 69 Optical mapping* 3.0 375 86-146 12 12 Laser altimetry# 2.2 304 15-75 5 5 High-res.images+ Ceres 11.0 1360 880-906 89 89 Optical mapping* 3.3 610 130-156 14 14 Laser altimetry# 2.7 529 49-75 6 6 High-res.images+

*Visible and IR; #Including gamma ray, magnetic and gravity data; +Shared control but generally nadir pointing; 1 framing camera

An equal amount of data is taken with the copy of that designed for the MESSENGER mapping spectrometer. After two weeks the orbiter. The gamma ray and neutron spacecraft is lowered to a laser altimetry and spectrometer has much heritage from the gamma ray optimized orbit with an altitude of spectrometer carried on Lunar Prospector. The about 120 km, where magnetic and gravity data magnetometer is a copy of that built for the ST5 are also obtained. After about 6 months, if safe, mission configured as a dual-sensor gradiometer the orbit is lowered further and high resolution along the 5 m boom. A third new technology imagery and spectra are taken. Orbital sensor is carried to provide redundancy and to parameters from these are given in Table 3 test a new method of measuring the magnetic for both Ceres and Vesta. field.

5. INSTRUMENTATION AND SCIENCE 6. CONCLUDING REMARKS OBJECTIVES The Dawn mission completes the first-order The top level objectives of the Dawn mission are exploration of the inner solar system. It also to understand the conditions and processes promises to surprise us. While our telescopes and occurring at the earliest moments in the solar studies have told us much about Vesta system and to determine the role of water in and something about Ceres, today we know very controlling planetary evolution. To do this Dawn little compared to what we hope to achieve with investigates the internal structure, density, and Dawn. homogeneity of two complementary bodies, Ceres and Vesta, that have remained intact from 7. ACKNOWLEDGEMENTS the time of their formation, by measuring their mass, shape, volume and spin state with imagery, laser altimetry and gravity. Dawn records the We would like to thank W.K. Hartmann for 's remanent magnetization, elemental permission to use his painting, "A cocoon and mineral composition to determine their nebula, perhaps the primordial solar nebula" in history and evolution. It provides context for the association with the Dawn mission. We also meteorites associated with Vesta and looks for wish to thank the many people who helped with evidence of processes that might explain the the Dawn proposals, in particular M. Shirbacheh, origin of primitive, but aqueously altered, C. Henry, G. Vane and A. McGlynn. The meteorites. Dawn images Ceres' and Vesta's preparation of this paper was supported by a surfaces to determine their bombardment and grant from the Discovery project office. tectonic history; uses gravity, spin state, and magnetic data to limit the size of any metallic 8. REFERENCES core; and uses infrared and gamma ray spectrometry to search for water bearing 1. Gaffey, M.J., et al., in Remote Geochemical minerals. The framing camera covers the spectral Analysis (ed. C.M. Pieters and P.A.J. Englert) range from 400 nm to 1000 nm with 7 color filter 43-77, CUP, New York, 1993. bands and one clear filter. The mapping 2. Rivkin, A.S., Observations of main-belt spectrometer covers the visible range to 5 asteroids in the 3 micron region, Ph.D. Thesis, microns in the infrared. The laser altimeter is a U of Arizona, 1997.